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[emscripten] Simply LTO flag setup. NFC #7997

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Merged
sbc100 merged 1 commit into from
Oct 28, 2025
Merged

[emscripten] Simply LTO flag setup. NFC #7997

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24 changes: 13 additions & 11 deletions CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,14 @@ endif()
# more useful error reports from users.
option(BYN_ENABLE_ASSERTIONS "Enable assertions" ON)

option(BYN_ENABLE_LTO "Build with LTO" ${EMSCRIPTEN})
if(EMSCRIPTEN AND NOT "${CMAKE_BUILD_TYPE}" MATCHES "Debug")
# in opt builds, LTO helps so much (>20%) it's worth slow compile times,
# so we enable LTO by default in non-debug emscripten builds.
set(DEFAULT_LTO ON)
else()
set(DEFAULT_LTO OFF)
endif()
option(BYN_ENABLE_LTO "Build with LTO" ${DEFAULT_LTO})

# Turn this off to avoid the dependency on gtest.
option(BUILD_TESTS "Build GTest-based tests" ON)
Expand Down Expand Up @@ -222,6 +229,11 @@ if(BYN_ENABLE_LTO)
set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=2)
set(CMAKE_JOB_POOL_LINK link_job_pool)
add_compile_flag("-flto=thin")
if(EMSCRIPTEN)
# Under emscripten we also add `-flto` to the linker flags to tell the
# compiler driver to choose LTO versions of the system libraries.
add_link_flag("-flto=thin")
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endif()
endif()

if(MSVC)
Expand Down Expand Up @@ -353,10 +365,6 @@ if(EMSCRIPTEN)
# On Node.js, make the tools immediately usable.
add_link_flag("-sNODERAWFS")
endif()
if (BYN_ENABLE_LTO)
# in opt builds, LTO helps so much (>20%) it's worth slow compile times
add_nondebug_compile_flag("-flto")
endif()
if(EMSCRIPTEN_ENABLE_WASM64)
add_compile_flag("-sMEMORY64")
add_link_flag("-sMEMORY64")
Expand Down Expand Up @@ -556,9 +564,6 @@ if(EMSCRIPTEN)
target_link_libraries(binaryen_wasm PRIVATE optimized "--closure=1")
# TODO: Fix closure warnings! (#5062)
target_link_libraries(binaryen_wasm PRIVATE optimized "-Wno-error=closure")
if (BYN_ENABLE_LTO)
target_link_libraries(binaryen_wasm PRIVATE optimized "-flto")
endif()
target_link_libraries(binaryen_wasm PRIVATE debug "--profiling")
# Avoid catching exit as that can confuse error reporting in Node,
# see https://github.com/emscripten-core/emscripten/issues/17228
Expand Down Expand Up @@ -610,9 +615,6 @@ if(EMSCRIPTEN)
endif()
# TODO: Fix closure warnings! (#5062)
target_link_libraries(binaryen_js PRIVATE optimized "-Wno-error=closure")
if(BYN_ENABLE_LTO)
target_link_libraries(binaryen_js PRIVATE optimized "-flto")
endif()
target_link_libraries(binaryen_js PRIVATE debug "--profiling")
target_link_libraries(binaryen_js PRIVATE debug "-sASSERTIONS")
# Avoid catching exit as that can confuse error reporting in Node,
Expand Down
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